Underwater pipelines comprise a number of pipes joined to total lengths of hundreds of kilometers. The pipes are of normally 12-metre standard length, and relatively large diameters ranging between 0.2 and 1.5 metres, and each comprise a steel cylinder; a first coating of polymer material to protect the steel pipe; and possibly a second coating of Gunite or cement to weigh down the pipeline. In some applications, the pipes and underwater pipelines do not need and therefore have no second coating.
To weld the steel cylinders to one another, the opposite free ends of each pipe have no first or second coating. The pipes are joined at on-land installations into multiple-standard-length pipes, as well as on pipeline-laying vessels, on which standard-length or multiple-standard-length pipes are joined to others, in turn already joined to other pipes, to form part of the underwater pipeline.
The actual joining operation comprises welding the steel cylinders, normally in a number of weld passes, and bridging the first and, possibly, second coating. Once an annular weld is formed between two steel cylinders, the cutback, with no first or second coating, extends astride the weld. In other words, the cutback is defined substantially by the free ends of the pipes, extends axially between two end portions of the first coating, and must be protective coated to prevent corrosion.
U.S. Pat. No. 3,470,057 discloses an old technique consisting in winding about the cutback a wrapping material composed of an outer layer of asphalt-impregnated layer or tar-impregnated asbestos and an inner layer of glass fiber embedded in a material such as thermoplastic tar or asphalt. The inner layer is sufficiently thermoplastic so as to be rendered thermoplastic by application of heat.
Cutback protective coating is known as “field joint coating”, and comprises coating the cutback with three coats to ensure protection and adhesion of the coats to the steel cylinders. More specifically, cutback protective coating comprises heating, e.g. induction heating, the cutback to 250° C.; spraying the cutback with powdered epoxy (FBE—Fusion Bonded Epoxy) resin, which, in contact with the cutback; forms a relatively thin first coat or “primer”; spraying the cutback, on top of the first coat, with a modified copolymer, which acts as adhesive and, in contact with the first coat, forms a relatively thin second coat; applying a third so-called “top coat”, which also extends partly over the first coating; and then bridging the second coating if necessary.
Welding, non-destructive weld testing, and bridging the first and second coating, are performed at joining stations equally spaced along the path of the pipes (or of the pipeline being formed, when the pipes are joined to this). The pipes are therefore advanced in steps, and are stopped at each joining station for a length of time determined by the longest operation, which, at present, is that of applying the third or top coat.
Known methods currently employed to apply the third coat include:                “cigarette wrapping”, which comprises heating, winding, and compressing a number of thin sheets of polymer material about the cutback, on top of the adhesive second coat;        “spiral wrapping”, which comprises heating, double-winding, and compressing a strip about the cutback, on top of the second coat;        “flame spraying” using a hot spray gun to melt and spray on polymer;        fitting a mold about the cutback, and injecting liquid polymer about the cutback, on top of the second coat;        preparing a polymer strip having a heat-shrink outer protective layer (third coat) and an adhesive inner layer (second coat); heat-shrinking the strip; and melting the adhesive inner layer so the strip adheres firmly to the first coat. This last method differs from the previous methods by simultaneously applying the second and third coat.        
All the above methods of applying the third coat are extremely time-consuming. More specifically, coating large cutbacks, such as those of a 48-inch (roughly 1.2-metre) diameter steel cylinder, calls for applying a relatively long third coat, which, in addition, may be as much as 5 mm thick and 400 mm wide. In other words, since, in most cases, the mass of polymer material to be applied to form the third coat is relatively considerable, and the third coat must be plastic enough, when applied, to achieve effective chemical/mechanical adhesion to the second coat, known methods of applying the third coat do not allow a satisfactory reduction in coating time.
Moreover, when applying the third coat using known methods, small amounts of air may become trapped between the third coat and underlying coats, and are particularly harmful by preventing firm grip of the third coat to the underlying coats, and possibly impairing the field joint coating as a whole.